STRUCTURE OF ARTERIES. 



365 



more open and lax towards the surface of the artery, where they connect the 

 vessel with its sheath or with other surrounding parts. Longitudinally arranged 

 contractile fibre-cells have been described by various observers in the external coat of 

 some arteries (e.g., the iliacs, superior mesenteric, splenic, renal, dorsalis penis, and the 



Fig. 424. SECTION OP THORACIC AORTA, AS 



SEEN UNDER A LOW POWER. (Toldt. ) 



a, inner coat, showing in its inner part longi- 

 tudinal muscular fibres cut across ; b, middle 

 coat, showing elastic membranes alternating with 

 the muscular tissue ; c, outer coat, with two sec- 

 tions of Vcisa vasorum. 



umbilical arteries of the foetus). In the 

 umbilical arteries, according to Eberth, 

 a complete layer of longitudinal mus- 

 cular fibres is also present in the middle 

 coat, internal to the ordinary circular 

 fibres. Scattered longitudinal muscular 

 cells are present in some arteries 

 amongst the circularly disposed fibres 

 of the middle coat, and even in the 

 subendothelial layer of the internal coat 

 (see fig. 424, in the aorta). The outer 

 ccJat is usually of greater proportionate 



thickness in the smaller arteries, but as it shades off into the surrounding connective 

 tissue it is difficult to adjudge its exact thickness. 



Some arteries have much thinner coats than the rest, in proportion to their 

 calibre. This is strikingly the case with those contained within the cavity of the 

 cranium, and in the vertebral canal ; the difference depends on the external and 

 middle coats, which in the vessels referred to are thinner than elsewhere. The 

 pulmonary arteries have also much thinner coats than those of the aortic system. 



Vessels and nerves of arteries. The coats of arteries receive small vessels, 

 both arterial and venous, named rasa vasorum, which serve for their nutrition. 

 The little nutrient arteries are not derived immediately from the cavity of the main 

 vessel but pass into its coats from branches which arise from the artery (or some- 

 times from a neighbouring artery), at some distance from the point where they are 

 ultimately distributed, and divide into smaller branches within the sheath, and upon 

 the surface of the vessel, before entering the outer coat where they are distributed 

 (fig. 424). In some of the larger mammals, a few pass into the middle coat, and 

 follow the circular course of its fibres, but in health none penetrate into this coat in 

 the human subject and still less into the internal coat (Ranvier). Minute venules 

 return the blood from these nutrient arteries, which, however, they do not closely 

 accompany, and discharge it into the vein or pair of veins which usually runs along- 

 side the artery. Lymphatics are present in the outer coat. 



Arteries are generally accompanied by larger or smaller nerves ; and when, in the 

 operation of tying an artery, these happen to be included along with it in the liga- 

 ture, pain may be experienced ; but the vessel itself, when in a healthy condition, is 

 insensible. Nerves are, nevertheless, distributed to the coats of arteries. They 

 form plexuses round the larger arteries, and run along the smaller branches in form 

 of fine bundles of fibres, which here and there twist round the vessel, and unite 

 with one another in a plexiform manner. The fine branches destined for the artery 

 penetrate to the middle coat, to the muscular tissue of which they are chiefly 

 distributed. 



B B 2 



